Cisco 3504 Wireless Controller 1 AP Adder License RTU (PDF) License . will email you the PDF license after buying . 

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https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst2960cx_3650cx/software/release/15-2_7_e/configuration_guide/b_1527e_consolidated_3560cx_2960cx_cg/m_sm_rtu_cg.pdf

LIC-CT3504-1A

Cisco 3504 Wireless Controller 1 AP Adder License.